Effects of sintering temperature and Bi2O3 content on microstructure and magnetic properties of LiZn ferrites

The effects of sintering temperature and Bi₂O₃ content on the microstructure and magnetic properties of lithium–zinc (LiZn) ferrites prepared by a conventional ceramic method were investigated. The results show that the densification behavior and grain growth rate were greatly improved by the addition of Bi₂O₃, because a liquid phase sintering occurred during the sintering process at high temperature due to the low-melting point of Bi₂O₃ (825 °C). X-ray diffraction (XRD) patterns of the slightly doped samples did not reveal the appearance of any phase other than spinel LiZn ferrite. However, the secondary phase of perovskite BiFeO₃ was detected for Bi₂O₃ content of more than 0.25 wt%. The studies further show that Bi oxide was present at grain boundary, and promoted the grain growth as reaction center at lower temperature. A high saturation magnetization, squareness ratio, minimum ferromagnetic resonance linewidth and low coercive force were obtained for the sample with 1.00 wt% Bi₂O additive at lower sintering temperature (1100 °C).     

Enhanced luminescence of Dy in Y3Al5O12 by Bi co-doping

In the present paper, phosphors with the composition Y_3−x−yAl₅O₁₂:Bi³⁺_x, Dy³⁺_y were synthesized with solid state reactions. The luminescence properties of Bi³⁺ and Dy³⁺ in Y₃Al₅O₁₂(YAG) and the energy transfer from Bi³⁺ to Dy³⁺ were investigated in detail. Bi³⁺ in YAG emits one broad band peaking at 304 nm which can be ascribed to the transition from excited states ³P_{0, 1} to ground state ¹S₀. Dy³⁺ in YAG emits two groups of peaks around 484 and 583 nm, respectively, which can be ascribed to the transitions from excited state ⁴F_9/2 to ground states ⁶H_15/2 and ⁶H_13/2. The co-doping of Bi³⁺ enhances the luminescent intensity of Dy³⁺ by ∼7 times because Bi³⁺ can transfer the absorbed energy to Dy³⁺ efficiently. The mechanism of energy transfer was also discussed.     

Effect of composition on the conductivity of CTAB-butanol-octane-nitrate salts (Al(NO3)3 + Zn(NO3)2) microemulsions and on the surface and textural properties of resulting spinels ZnAl2O4

The conductivity σ of a microemulsion series consisting of CTAB + butanol + octane, in which a solution of Al(NO₃)₃ 0.8 M + Zn(NO₃)₂ 0.4 M was gradually added, was studied at room temperature as a function of its composition φ. The addition of nitrate salts solution took place in four different ratios of (butanol + CTAB):octane = 0.2, 0.4, 0.6 and 0.8. Initially, all those four systems are (water in oil, w/o) microemulsions and the gradual addition of the solution of the nitrate salts transforms them to bicontinuous ones. The conductivity increases gradually, but with different rate in each case, and the corresponding critical exponents at the percolation threshold were determined from the curves σ = f(φ). Next at three different compositions of microemulsions, corresponding to ratios (butanol + CTAB):octane = 0.4, 0.6 and 0.8 and ratio of the nitrate salts solution x ≈ 0.25, spinels ZnAl₂O₄ were isolated/prepared. XRD, SEM and N₂ adsorption-desorption measurements were used to determine the structure and texture of those solids. From those measurements the surface area (S_p), the pore volume (V_p), the size of crystallites and the average pore connectivity (c) were found. Those properties showed considerable variation and dependence on the composition of the original microemulsions employed in the preparation, a fact indicating that the structure and texture of the obtained solids can be manipulated at will via the composition of microemulsion used.     

Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Binary (ZnO)_0.5(P₂O₅)_0.5 glasses doped with Eu₂O₃ and nanoparticles of Gd₂O₃:Eu were prepared by conventional melt-quench method and their luminescence properties were compared. Undoped (ZnO)_0.5(P₂O₅)_0.5 glass is characterized by a luminescent defect centre (similar to L-centre present in Na₂O-SiO₂ glasses) with emission around 324 nm and having an excited state lifetime of 18 ns. Such defect centres can transfer the energy to Eu³⁺ ions leading to improved Eu³⁺ luminescence from such glasses. Based on the decay curves corresponding to the ⁵D₀ level of Eu³⁺ ions in both Gd₂O₃:Eu nanoparticles incorporated as well as Eu₂O₃ incorporated glasses, a significant clustering of Eu³⁺ ions taking place with the latter sample is confirmed. From the lifetime studies of the excited state of L-centre emission from (ZnO)_0.5(P₂O₅)_0.5 glass doped with Gd₂O₃:Eu nanoparticles, it is established that there exists weak energy transfer from L-centres to Eu³⁺ ions. Poor energy transfer from the defect centres to Eu³⁺ ions in Gd₂O₃:Eu nanoparticles doped (ZnO)_0.5(P₂O₅)_0.5 glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between luminescent centre and Eu³⁺ ions.     

Influence of valence states and co-ordination of cobalt ions on dielectric properties of PbO-Bi2O3-As2O3:CoO glass system

The glasses of the composition (40−x) PbO-15Bi₂O₃-45As₂O₃-xCoO, with 0≤x≤0.6 mol% in the steps of 0.1 were synthesized. The dielectric properties viz., dielectric constant, loss and ac conductivity over moderately larger ranges of frequency and temperature were investigated. The results were analyzed with the aid of the data on optical absorption and IR spectra. The analysis indicated that there is an increase in the insulating strength of the glasses with increase in the concentration of CoO up to 0.4 mol%.     

Effect of BiFeO3 addition on Bi2O3–ZnO–Nb2O5 based ceramics

In this paper, low temperature sintering of the Bi₂(Zn_1/3Nb_2/3)₂O₇ (β-BZN) dielectric ceramics was studied with the use of BiFeO₃ as a sintering aid. The effects of BiFeO₃ contents and the sintering temperature on the phase structure, density and dielectric properties were investigated. The results showed that the sintering temperature could be decreased and the dielectric properties could be retained by the addition of BiFeO₃. The structure of BiFeO₃ doped β-BZN was still the monoclinic pyrochlore phase. The sintering temperature of BiFeO₃ doped β-BZN ceramics was reduced from 1000 °C to 920 °C. In the case of 0.15 wt.% BiFeO₃ addition, the β-BZN ceramics sintered at 920 °C exhibited good dielectric properties, which were listed as follows: ε_r = 79 and tan δ = 0.00086 at a frequency of 1 MHz. The obtained properties make this composition to be a good candidate for the LTCC application.     

Effects of Li2CO3 on the sintering behavior and piezoelectric properties of Bi2O3-excess (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics doped with Li₂CO₃ and Bi₂O₃ as sintering aids were manufactured, and their micro structural, dielectric and piezoelectric properties were investigated. All specimens could be well sintered at a low-temperature of 1080 °C. The bulk density of the specimens doped with a small amount of Li₂CO₃ was enhanced. The dielectric and piezoelectric properties of ceramics were investigated with different amounts of Li₂CO₃ substitutions. High electrical properties of d₃₃ = 167 pC/N, k_p = 0.34, P_r = 40 μC/cm² and E_c = 38 kV/cm were obtained from the specimen containing 0.1 mol% of Li₂CO₃ sintered at 1080 °C.     

Synthesis and magnetic properties of hard magnetic (CoFe2O4)-soft magnetic (Fe3O4) nano-composite ceramics by SPS technology

CoFe₂O₄/Fe₃O₄ nano-composite ceramics were synthesized by Spark Plasma Sintering. The X-ray diffraction patterns show that all samples are composed of CoFe₂O₄ and Fe₃O₄ phases when the sintering temperature is below 900 °C. It is found that the magnetic properties strongly depend on the sintering temperature. The two-step hysteresis loops for samples sintered below 500 °C are observed, but when sintering temperature reaches 500 °C, the step disappears, which indicates that the CoFe₂O₄ and Fe₃O₄ are well exchange coupled. As the sintering temperature increases from 500 to 800 °C, the results of X-ray diffractometer indicate the constriction of crystalline regions due to the ion diffusion at the interfaces of CoFe₂O₄/Fe₃O₄ phases, which have great impact on the magnetic properties.     

Effects of different sintering temperature and Nb2O5 content on structural and magnetic properties of Z-type hexaferrites

Nb-doped Z-type hexaferrites (Ba₃(Co_0.4Zn_0.6)₂Fe₂₄O₄₁) with composition of Ba₃(Co_0.4Zn_0.6)₂Fe₂₄O₄₁+x Nb₂O₅ (where x=0.0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.2, 1.6 and 2.0 wt%) were prepared by a solid-state reaction method. The effects of different sintering temperature (T_s) and Nb₂O₅ content on the sintering behaviors, phase composing, microstructure, and magnetic properties of the samples were investigated. The results from X-ray diffraction and scanning electron microscopy show that as the amount of Nb₂O₅ additive increases, the major phase changes to Z-phase, Simultaneously, M-phase and a small amount of niobate phase appear. The Nb₂O₅ additive promotes the grain growth as reaction center at lower sintering temperature (1220 °C), but at higher temperature (1260 °C), niobate phase separated out in grain boundaries as secondary phase will restrain abnormal grain growth, so closed pores in grains are not formed. The Nb₂O₅ additive can enhance densification, improve initial permeability of hexaferrites by increasing the grain growth of hexaferrite and the displacement of ions in the sintering process due to the aberration and activation of crystal lattice, which is accompanied by the solubility of Nb⁵⁺ in the hexaferrites. A relative density of 96%, maximum initial permeability (32–33), minimum coercivity (454–455 A/m) and resonance frequency above 400 MHz were obtained for the sample with 0.8 wt% Nb₂O₅ sintered at 1260 °C for 6 h.     

Formation of highly textured (1 1 1) Bi2O3 films by anodization of electrodeposited bismuth films

Highly textured bismuth oxide (Bi₂O₃) thin films have been prepared using anodic oxidation of electrodeposited bismuth films onto stainless steel substrates. The Bi₂O₃ films were uniform and adherent to substrate. The Bi₂O₃ films were characterized for their structural and electrical properties by means of X-ray diffraction (XRD), electrical resistivity and dielectric measurement techniques. The X-ray diffraction pattern showed that Bi₂O₃ films are highly textured along (1 1 1) plane. The room temperature electrical resistivity of the Bi₂O₃ films was 10⁵ Ω cm. Dielectric measurement revealed normal oxide behavior with frequency.     

Chemical quenching of positronium in Fe2O3/Al2O3 catalysts

Fe₂O₃/Al₂O₃ catalysts were prepared by solid state reaction method using α-Fe₂O₃ and γ-Al₂O₃ nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes τ₃ and τ₄ are attributed to positronium annihilation in two types of pores distributed inside Al₂O₃ grain and between the grains, respectively. With increasing Fe₂O₃ content from 3 wt% to 40 wt%, the lifetime τ₃ keeps nearly unchanged, while the longest lifetime τ₄ shows decrease from 96 ns to 64 ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe₂O₃ content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of τ₄ is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores.     

Enhanced luminescence of Ca3B2O6:Dy phosphors by Na-codoping for LED applications

The Ca_2.95−yDy_0.05B₂O₆:yNa⁺ (0≤y≤0.20) phosphors were synthesized at 1100 °C in air by the solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), photoluminescence excitation (PLE), photoluminescence (PL) spectra and thermoluminescence (TL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid-state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions on 350 nm excitation was observed at 480 nm (blue) due to the ⁴F_9/2→⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2→⁶H_13/2 transitions and 660 nm (red) due to weak ⁴F_9/2→⁶H_11/2 emissions. The PL results from the investigated Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors show that Dy³⁺ emissions increase with the increase of the Na⁺ codoping ions. The integral intensity of yellow to blue (Y/B) can be tuned by controlling Na⁺ content. By the simulation of white light, the optimal CIE value (0.328, 0.334) can be achieved when the content of Na⁺-codoping ions is y=0.2. The results imply that the Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors could be potentially used as white LEDs.     

Photoluminescence properties and effects of dopant concentration in Bi2ZnB2O7:Tb phosphor

A novel green phosphor, Tb³⁺ doped Bi₂ZnB₂O₇ was synthesized by conventional solid state reaction method. The phase of synthesized materials was determined using the XRD, DTA/TG and FTIR. The photoluminescence characteristics were investigated using spectrofluorometer at room temperature. Bi₂ZnB₂O₇:Tb³⁺ phosphors excited by 270 nm and 485 nm wavelengths. The emission spectra were composed of three bands, in which the dominated emission of green luminescence Bi₂ZnB₂O₇:Tb³⁺ attributed to the transition ⁵D₄ → ⁷F₅ is centered at 546 nm. The dependence of the emission intensity on the Tb³⁺ concentration for the Bi_2−xTb_xZnB₂O₇ (0.01 ≤ x ≤ 0.15) was studied and observed that the optimum concentration of Tb³⁺ in phosphor was 13 mol% for the highest emission intensity at 546 nm.     

Low temperature fired Ni-Cu-Zn ferrite with Bi4Ti3O12

The Ni-Cu-Zn ferrites with different contents of Bi₄Ti₃O₁₂ ceramics (1-8 wt%) as sintering additives were prepared by the usual ceramic technology and sintered at 900 °C to adapt to the low temperature co-fired ceramic (LTCC) technology. The magnetic and dielectric properties of the ferrite can be effectively improved with the effect of an appropriate amount of Bi₄Ti₃O₁₂. For all samples, the ferrite sintered with 2 wt% Bi₄Ti₃O₁₂ has relatively high density (98.8%) and permeability, while the ferrite with 8 wt% Bi₄Ti₃O₁₂ has relatively good dielectric properties in a wide frequency range. The influences of Bi₄Ti₃O₁₂ addition on microstructure, magnetic and dielectric properties of the ferrite have been discussed.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Exchange bias in thin-film (Co/Pt)3/Cr2O3 multilayers

We have fabricated exchange-biased Co/Pt layers ((0.3 nm/1.5 nm)×3) on (0 0 1)-oriented Cr₂O₃ thin films. The multilayered films showed extremely smooth surfaces and interfaces with root mean square roughness of ≈0.3 nm for 10 μm×10 μm area. The Cr₂O₃ films display sufficient insulation with a relative low leakage current (1.17×10⁻² A/cm² at 380 MV/m) at room temperature which allowed us to apply electric field as high as 77 MV/m. We find that the sign of the exchange bias and the shape of the hysteresis loops of the out-of-plane magnetized Co/Pt layers can be delicately controlled by adjusting the magnetic field cooling process through the Néel temperature of Cr₂O₃. No clear evidence of the effect of electric field and the electric field cooling was detected on the exchange bias for fields as high as 77 MV/m. We place the upper bound of the shift in exchange bias field due to electric field cooling to be 5 Oe at 250 K.     

Effect of Nd concentration on structural and optical properties of Nd:Y2O3 transparent ceramic

Y₂O₃ transparent ceramics with different Nd concentration (0.1-7.0at%) were fabricated using ZrO₂ as additive. All the samples exhibit high transparency over a broad spectral region. The elements (Y, O and Nd) are uniformly distributed in the ceramic body, and the average grain size increases with Nd content. Based on the absorption spectrum, the Judd-Ofelt intensity parameters are calculated (Ω₂=4.364×10⁻²⁰ cm², Ω₄=3.609×10⁻²⁰ cm² and Ω₆=2.919×10⁻²⁰ cm²). The absorption coefficients increase linearly with Nd³⁺ doping concentration. The absorption cross-section at 804 nm and stimulated emission cross-section at 1078 nm are calculated to be 1.54×10⁻²⁰ and 7.24×10⁻²⁰ cm², respectively. All the emission bands exhibit the highest emission intensities with 1.0at% Nd³⁺ ion content, while the lifetime decreases dramatically from 321.5 μs (0.1at% Nd) to 17.9 μs (7.0at% Nd). According to the emission spectra and measured lifetime, the optimum doping concentration of Nd³⁺ ion in Y₂O₃ transparent ceramic might be around 1.0at%.     

Improving microwave dielectric properties of La2.98/3Sr0.01(Mg0.5Sn0.5)O3 ceramics with CuO additive

The microwave dielectric properties of CuO-doped La_2.98/3Sr_0.01(Mg_0.5Sn_0.5)O₃ ceramics were investigated with a view to their application in microwave devices. CuO-doped La_2.98/3Sr_0.01(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of CuO-doped La_2.98/3Sr_0.01(Mg_0.5Sn_0.5)O₃ ceramics exhibited no significant variation of phase with sintering temperature. By adding 0.75 wt.% CuO, a dielectric constant of 20.07, a quality factor (Q × f) of 63,000 GHz, and a temperature coefficient of resonant frequency τ_f (−77.0 ppm/°C) were obtained when La_2.98/3Sr_0.01(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1500 °C for 4 h.     

Photoluminescence properties of Eu in Y(PO3)3 under VUV excitation

The photoluminescence properties of Y_1−x(PO₃)₃:xEu³⁺ (0<x≤0.2) are investigated. The excitation spectrum of Y_0.85(PO₃)₃:0.15Eu₃⁺ shows that both the (PO₃)₃³⁻ groups and the CT bands of O²⁻-Y³⁺ can efficiently absorb the excitation energy in the region of 120-250 nm. Under 147 nm excitation, the optimal emissive intensity of Y_1−x(PO₃)₃:xEu³⁺ (0<x≤0.2) is about 36% of the commercial phosphor (Y,Gd)BO₃:Eu³⁺, which hints that the absorbed energy by the host matrix could be efficiently transferred to Eu³⁺. We try to study the concentration quenching mechanism of Y_1−x(PO₃)₃:xEu³⁺ (0<x≤0.2) under 147 and 172 nm excitation.     

Crystal structure and photoluminescence of (Y1−xCex)2Si3O3N4

Oxonitridosilicate phosphors with compositions of (Y_1−xCe_x)₂Si₃O₃N₄ (x=0−0.2) have been synthesized by solid state reaction method. The structures and photoluminescence properties have been investigated. Ce³⁺ ions have substituted for Y³⁺ ions in the lattice. The emission and excitation spectra of these phosphors show the characteristic photoluminescence spectra of Ce³⁺ ions. Based on the analyses of the diffuse reflection spectra and the PL spectra, a systematic energy diagram of Ce³⁺ ion in the forbidden band of sample with x=0.02 is given. The best doping Ce content in these phosphors is ∼2 mol%. The quenching temperature is ∼405 K for the 2 mol% Ce content sample. The luminescence decay properties were investigated. The primary studies indicate that these phosphors are potential candidates for application in three-phosphor-converted white LEDs.